The invention relates generally to wavelength division multiplexed transmission systems, and more particularly to a method and apparatus for performing dispersion compensation without a change in polarization.
Wavelength-division multiplexing is expected to be increasingly utilized in undersea and transcontinental terrestrial optical transmission systems due, in part, to the large bandwidth capacity such multiplexing provides. One way of increasing the total transmission capacity of such systems is to more efficiently use the available spectral bandwidth such as by decreasing the spacing between adjacent ones of the multiplexed channels. Unfortunately, wavelength division multiplexed transmission systems are susceptible to performance limitations due to polarization dependent effects such as cross-talk between the multiplexed channels. Cross-talk, which is primarily caused by the non-linear index of refraction of optical transmission fibers, increases as the channel spacing decreases. Four-wave mixing is one significant deleterious effect that produces cross-talk
U.S. Application Serial No. [N.S. Bergano 22-4] entitled xe2x80x9cMethod and Apparatus for Improving Spectral Efficiency in Wavelength Division Multiplexed Transmission Systems,xe2x80x9d discloses an optical transmitter that generates a WDM signal having even-numbered channels in a state of polarization (SOP) orthogonal to the SOP of the odd-numbered channels. This arrangement advantageously limits the four-wave mixing products that can be generated in the transmitter and the optical transmission path to which it is typically coupled.
Wavelength division multiplexed systems must also employ dispersion management techniques. As the per channel data rates of such system increase, the interplay of dispersion and fiber nonlinearity needs to be more carefully managed. Typically the transmission line is designed to have an average dispersion value of zero. In the case of WDM systems, however, only one channel can be arranged to have an average dispersion of zero. The remaining channels will have some net nonzero dispersion due to the dispersion slope of the optical fibers forming the transmission One technique for overcoming this limitation in WDM systems is to compensate for those channels that do not have a net zero dispersion by adding compensating dispersion fiber at the receiver or transmitter so that their net accumulated dispersion is zero. It has been shown to be advantageous to provide a portion of the dispersion compensation at the transmitter and a portion at the receiver.
While it would be beneficial to provide a transmitter that offers both dispersion compensation and a signal in which adjacent channels have orthogonal SOPs, such a transmitter has not been available because it has not been possible to perform dispersion compensation in a polarization maintaining environment.
Accordingly, it would be desirable to provide a method and apparatus for performing dispersion compensation without changing the polarization of the optical signal.
In accordance with the present invention, a method and apparatus is provided for transmitting an optical signal. The method includes the step of generating an optical signal that includes a plurality of optical channels, which are sequentially numbered from 1 to N from lowest to highest wavelength. Dispersion compensation is imparted to each of the plurality of optical channels without a change in polarization of the channels. A state-of-polarization of predetermined odd-numbered channels is oriented to be substantially orthogonal to a state of polarization of predetermined even-numbered channels by directing the predetermined odd-numbered channels and the predetermined even-numbered channels through orthogonally polarizing inputs of a polarization coupler. The odd-numbered channels and the even-numbered channels may be directed through first and second wavelength combiners, respectively, prior to orienting their states of polarization. The orthogonal relationship between the states of polarization of odd and even-numbered channels advantageously limits the four-wave mixing products that can be generated in the optical transmitter and the optical transmission path to which it is typically coupled.
In one embodiment of the invention, dispersion compensation is provided by device that includes an optical circulator having an input port, an intermediate port, and an output port. First and second polarization maintaining optical fibers are respectively coupled to the input and output ports. The first and second polarization maintaining fibers have an orientation offset by 90 degrees with respect to one another. A single mode fiber is coupled at one end to the intermediate port and at the other end to a dispersion compensating fiber. A Faraday rotator mirror is coupled to the dispersion compensating fiber. Such a device provides dispersion compensation without a change in polarization.
In another embodiment of the invention, the dispersion compensator includes an optical circulator having an input port, an intermediate port, and an output port. First, second and third polarization maintaining optical fibers are respectively coupled to the input, intermediate and output ports. A Faraday rotator is coupled at one end to the second polarization maintaining fiber and at another end to a chirped fiber grating.